The Global System for Mobile (GSM) telecommunications is used in cellular telephone networks in many countries around the world. GSM offers a useful range of network services and standards. Existing GSM networks are based on time-division multiple access (TDMA) digital communications technology, but an evolution is taking place toward the use of code-division multiple access (CDMA) technology. The present invention is applicable to both existing and future GSM networks, and although terminology used in the present patent application relates to existing GSM standards, it will be understood that the present invention is in no way limited to currently-existing standards or networks.
Handover in cellular systems can take place either while the MS is operating in a dedicated mode, i.e., during a telephone call, or while the MS is in an idle mode, between calls. The objective of the idle mode handover is to have the MS “camped on” the most appropriate base station cell of the network (i.e., tuned to the cell's control channels in order to receive paging and broadcast signals therefrom), and registered in the network via that cell. The MS is then ready to initiate or receive service requests immediately upon demand.
The GSM family of standards, and in particular, GSM standard 03.22, which is incorporated herein by reference, define idle mode operation in terms of three major, interrelated processes:                Selection of a public land mobile network (PLMN);        Cell selection and reselection; and        Location updating.        
In the protocol stack defined by GSM standards for signaling between the MS and base station, these idle mode functions are carried out by a radio interface protocol layer 3 (RIL-3). Within this layer, the cell selection/reselection process is carried out by a radio resource management (RR) sub-layer; and the PLMN selection and location updating processes are carried out by a mobility management (MM) sub-layer.
Every time the MS enters the idle mode (upon switch-on or upon termination of a call), it selects a PLMN and attempts to camp on a cell of that PLMN. The particular PLMN to be contacted may be selected either manually or automatically, according to a predefined order of priorities. The cell selection procedure verifies that the MS is camped on a suitable cell, in which it will be able to reliably receive and decode data and in which it is likely to be granted access when initiating a call. Cell selection may be based on a general search of all possible base station channels, or it may be aided by reference to a stored list of cells and/or cell signal strength measurements made while the MS was in dedicated mode. The alternative methods of cell selection are referred to in GSM parlance as follows:                Normal Cell Selection, wherein the MS searches all channels in all supported bands of operation;        Stored List Cell Selection, wherein the search is based on a stored list of cells in order to speed up the initial cell selection; and        Choose Cell Selection wherein the MS uses measurements made in dedicated mode in order to speed up cell selection after call termination.        
Once the MS has selected and camped on the appropriate cell, it sends a location update message to the base station so as to register its location with the network.
The MS continually monitors the signal received from the current cell on which it is camped, as well as the signals from neighboring cells. If a cell change is indicated, for example, because the signal from one of the neighboring cells is stronger than that of the current cell, or because of network conditions and priorities, cell reselection is invoked, followed by location update as required. If the MS loses contact with the current cell, cell selection and, if necessary, PLMN selection are likewise invoked.
If no suitable cell is found or the MS is not permitted to receive service (if, for example, the GSM subscriber identity module [SIM] is not properly inserted, or the network rejects a location update request), the MS enters a limited service mode. In limited service, the MS attempts to camp on any cell that will allow it to make emergency calls, irrespective of its PLMN identity.
Although the above description refers specifically to GSM standards and TDMA operation, handover and idle mode functions are a part of other cellular systems and standards, as well. Code-division multiple access (CDMA) is an improved digital communications technology, which affords more efficient use of radio bandwidth than TDMA, as well as a more reliable, fade-free link between cellular telephone subscribers and base stations. The CDMA standard that is currently deployed is TIA/EIA-95 (commonly referred to as IS-95), promulgated by the Telecommunications Industry Association (TIA).
GSM and CDMA standards of relevance to the present patent application are listed for reference in Appendix A at the end of this specification.
Hybrid GSM/CDMA cellular communications systems are described in the patent literature, although none have yet been commercially deployed. For example, PCT patent application PCT/US96/20764, which is incorporated herein by reference, describes a wireless telecommunications system that uses a CDMA air interface (i.e., basic RF communications protocols) to implement GSM network services and protocols. Using this system, at least some of the TDMA base stations (BSSs) and subscriber units of an existing GSM network would be replaced or supplemented by corresponding CDMA equipment. CDMA BSSs in this system are adapted to communicate with GSM mobile switching centers (MSCs) via a standard GSM A-interface. The core of GSM network services is thus maintained, and the changeover from TDMA to CDMA is transparent to users.
Hybrid cellular communications networks, incorporating both GSM and CDMA elements, are also described in PCT patent publications WO 95/24771 and WO 96/21999, and in an article by Tscha, et al., entitled “A Subscriber Signaling Gateway between CDMA Mobile Station and GSM Mobile Switching Center,” in Proceedings of the 2nd International Conference on Universal Personal Communications, Ottawa (1993), pp. 181–185, which are incorporated herein by reference. None of these publications deals with specific issues of how to implement efficient handovers and idle mode operation in such hybrid networks.
PCT patent application PCT/US97/00926, which is also incorporated herein by reference, describes methods of dedicated intersystem handover between CDMA and TDMA BSSs in a hybrid GSM/CDMA telecommunications system. A GSM/TDMA BSS generates pilot beacon signals in accordance with CDMA technology. During a telephone call, a subscriber unit detects the pilot signals and notifies a base station controller that the signals have been detected. The subscriber unit is then handed over from the CDMA to the TDMA BSS without interrupting the call.